The present invention relates to fiber amplifiers having filter means for attenuating or removing unwanted wavelengths and to optical couplers for use in fiber amplifiers and other optical systems.
Doped optical fiber amplifiers consist of a gain optical fiber the core of which contains a dopant such as rare earth ions. Such an amplifier receives an optical signal containing wavelength .lambda..sub.s and a pump signal containing wavelength .lambda..sub.p ; these signals are coupled to the gain fiber by means such as one or more couplers located at one or both ends of the amplifier. In the absence of an input signal, high levels of pump power can emanate from the gain fiber. Moreover, fiber amplifiers that are based on a three level laser system can radiate remnant pump light even when a signal is present, if they are designed for operation at maximum efficiency. In an Er-doped amplifier, for example, the remnant pump power can be 20 mW or higher when a 980 nm pump source is employed. This level can degrade the performance of the most widely used type of polarization insensitive optical isolators by heating up the YIG crystal of the Faraday rotators. There has been some evidence of destruction of isolators due to interaction of high power levels of light and optical epoxies. Operator safety also requires that 980 pump light be reduced to safe levels. It is therefore desirable to severely attenuate the unwanted pump light while propagating the signal light with essentially no attenuation.
Heretofore, fiber amplifiers have incorporated bulk optic elements such as isolators and filters for the purpose of protecting pump sources. See for example, the publication: M. Yoshida et al, "Development of Compact Er.sup.3+ -doped Fiber Amplifiers for Practical Applications", Topical Meeting of the Optical Society of America on Optical Amplifiers and Their Applications, Aug. 6-8, 1990, pp. 282/WD1-285/WD1. Such bulk optical devices introduce excessive loss and reflection into the system and can be sensitive to environmental conditions.